Transformer for use in a static inverter

Abstract

A novel and economical transformer is described for use in a static inverter in association with one or two switching devices, typically transistors. The transformer produces an output for control of the associated switching device(s) which changes in sense as a function of the flux level in the transformer core. The arrangement is applicable to a figure "8" flux configuration such as is achieved from two "E" cores, and requires only a single aperture located at the base of the common branch. Control is effected by a single primary and single secondary winding wound through the aperture. With two switching devices, two apertures are normally provided. The core which supports a figure "8" flux configuration may take other more economical alternatives such as "I" core, "T" core or a single "E" core.

What is claimed is: 
     
       1. A transformer comprising: A. a core of substantially linear magnetic material for main flux pursuing first and second closed magnetic paths arranged adjacently in a FIG. "8" configuration, a small aperture in said core disposed substantially at the boundary between flux paths for minimum interception of main flux, said closed paths separating at said aperture,   said aperture being bounded by three contiguous regions collectively forming a small virtual toroid, the first region also providing a portion of the path for main flux in said first closed path, the second region also providing a portion of the path for main flux in the second closed magnetic path, and the third region providing a path between said first and second regions substantially free of main flux in the absence of saturation effects,     B. a substantially helical power winding coupled to said core for generating and responding to main flux in said first and second closed flux paths,   C. feedback winding means for deriving a control output having a sense which reverses upon saturation of a predetermined toroidal region, said means comprising: (1) a primary feedback winding wound through said aperture to generate flux about said virtual toroid; and   (2) a secondary feedback winding wound through said aperture and embracing said third toroidal region for deriving a control output in response to flux therein.     
     
     
       2. A transformer as set forth in claim 1 wherein the reluctance of said toroid to flux generated by said primary feedback winding is smaller in the absence of saturation, any alternative flux paths, if said first or second region saturates, providing a substantially higher reluctance.   
     
     
       3. A transformer as set forth in claim 1 wherein said core extends along the axis of said power winding and comprising a center portion with a first end and a second end, said aperture being disposed near said first end.   
     
     
       4. A transformer as set forth in claim 3 wherein the reluctance of said toroid to flux generated by said primary feedback winding is smaller in the absence of saturation, saturation of said first or second region providing a virtual air gap and a horseshoelike magnetic structure of substantially higher reluctance.   
     
     
       5. A transformer as set forth in claim 3 wherein said core is extended at said one end to carry flux orthogonal to said axis symmetrically along with said first and second closed flux paths.   
     
     
       6. A transformer as set forth in claim 5 wherein said core has two linear portions joined to the orthogonal extensions of said center portion and extending parallel to said center portion.   
     
     
       7. A transformer as set forth in claim 3 wherein said core is tee-shaped to reduce flux extending axially beyond said one end, and permit mounting said one end on a metallic substrate with minimal eddy current losses.   
     
     
       8. A transformer as set forth in claim 7 wherein the reluctance of said toroid to flux generated by said primary feedback winding is smaller in the absence of saturation, saturation of said first or second region providing a virtual air gap and a horseshoelike magnetic structure of substantially higher reluctance.   
     
     
       9. A transformer as set forth in claim 1 wherein said core is a double E core arranged in a figure "8 " configuration.   
     
     
       10. A transformer as set forth in claim 9 wherein the reluctance of said toroid to flux generated by said primary feedback winding is smaller in the absence of saturation, saturation of said first or second region providing a virtual air gap, any alternative magnetic paths being of substantially higher reluctance than said unsaturated toroid.   
     
     
       11. A transformer as set forth in claim 10 wherein air gaps are provided at the joints of said double E core to increase the alternative path reluctance.   
     
     
       12. In combination with the transformer set forth in claim 1 means for supplying current to said power winding and to said primary feedback winding in mutually proportional amounts,   said primary feedback winding generating a circulating flux around said virtual toroid having a first direction of increase in said third region, the same direction of increase as the main flux in said first region, and an opposite direction of increase as the main flux in said second region, predisposing said first region to saturation prior to the second region when said current increases.   
     
     
       13. The combination as set forth in claim 12 wherein said feedback winding means and said three regions form a virtual current transformer, saturation of said one region reducing the effectiveness thereof.   
     
     
       14. The combination as set forth in claim 13 wherein said primary feedback winding embraces said third region.   
     
     
       15. The combination as set forth in claim 14 wherein with said first region saturated, said main power winding and said secondary feedback winding form a shunted virtual current transformer whose output is in a sense opposite to the sense of said virtual current transformer formed by said feedback winding means, additional main flux increases after saturation reversing the direction of flux change in said third region.   
     
     
       16. The combination set forth in claim 12 having in addition thereto: A. a switching transistor having a base input electrode and collector and emitter output electrodes, said secondary feedback winding being coupled between said base and emitter electrodes and maintaining a substantially constant voltage drop (V be ) across said secondary feedback winding when said transistor is conductive, constraining a substantially constant rate of change of flux (φ t ) in said secondary feedback winding:   φ.sub.t ˜(V.sub.be /n.sub.b)     where n b  are the turns of said secondary winding,     B. means coupling said primary winding and the output electrodes of said switching transistor in series across said current supply means to provide adequate current to saturate said first region,   said arrangement producing a conduction inducing base drive whose initial value is substantially equal to the turns ratio of the primary to secondary feedback winding times the transistor output current, and which decreases as a function of time in proportion to the reluctance associated with said feedback windings, the slope of said decrease being increased when said first region saturates, precluding appreciable incremental flux from flowing in said virtual toroid and forcing it to flow in a high reluctance path.   
     
     
       17. The combination as set forth in claim 16 wherein A. a second aperture is provided in said core similar to said first aperture but located at the opposite end of said core,   B. a second feedback winding means is provided similar to said first feedback winding means but wound through said second aperture,   C. a second switching transistor is provided similar to said first switching transistor but associated with said second feedback winding means, and   D. said power winding is center tapped, having one winding half serially coupled with the output electrodes of said first switching transistor in series across said current supply means and the second winding half being serially coupled with the output electrodes of said second switching transistor in series across said current supply means.   
     
     
       18. A transformer as set forth in claim 3 wherein said core is has a linear center portion shaped with a length substantially equal to the length of the axis of said power winding.   
     
     
       19. A transformer as set forth in claim 3 wherein said core is "  " shaped, fabricated from two "E" cores.   
     
     
       20. A transformer as set forth in claim 1 wherein said feedback winding means retains a current transformer action in feedback sense reversal in relation to a transistor input junction lead.